Integrated precise orbit determination for LEO constellation and BDS-3 MEO satellites using inter-satellite links and onboard BDS-3 observations

Abstract By leveraging the high-precision spatial reference established with Global Navigation Satellite System (GNSS), we propose a low-ground-dependency and low-latency Precise Orbit Determination (POD) method employing onboard GNSS, Inter-Satellite Link (ISL) observations and readily available GNSS broadcast ephemerides, thereby reducing the need for additional ground infrastructure in the construction of Low Earth Orbit (LEO) navigation augmentation systems. By combining ISL and GNSS data from LEO satellites, this method integrated estimates the orbits of both LEO and GNSS satellites, forming a high-low unified constellation. Due to the lack of absolute spatial reference, it is inevitably subject to a common systematic rotation. To correct this, we introduce an approach that estimates the rotation angles between the coordinate system implied in the integrated GNSS POD solutions and that of the broadcast ephemerides. These angles are then used to construct rotation correction matrices and remove the systematic rotation errors from the integrated POD solutions. We validate the method using 24 BeiDou-3 Satellite Navigation System (BDS-3) Medium Earth Orbit (MEO) satellites and a LEO constellation consisting of 66 LEO satellites. After correction, the along- and cross-track orbit errors of LEO constellation decrease from 22.7 cm and 39.3 cm to 1.3 cm and 4.2 cm, respectively; for BDS-3 MEO satellites, they reduced from 124.3 and 137.8 cm to 13.2 and 13.7 cm. However, some residual error remains due to the systematic rotation inherent in the broadcast ephemerides. When this is removed, Three-Dimensional (3D) accuracy improves from 4.4 to 1.0 cm for LEO satellites, and from 19.3 to 4.6 cm for MEO satellites. As the rotation has less effect on the radial component, radial errors remain at 0.2 cm for LEO satellites and 3.4 cm for MEO satellites. Additionally, we show that, thanks to ISL connectivity, accurate POD is achievable even when only a subset of LEO satellites carries GNSS receivers. Finally, we assess the impact of using predicted Earth Rotation Parameters (ERP), and find that ERP prediction errors mainly affect the rotation correction but less the integrated POD process.